I've just been playing TF2 Beta, and noticed the achievements I got showed up as being in US PST, when I'm in Australian EDT (/etc/localtime is Australia/NSW). If it helps, my hardware clock is in UTC.

In native Linux Steam, when I go to Community->Profile (using the integrated web browser) my achievements show up as ~2am, but if I close Steam and open up Windows Steam in Wine, the same achievements show up as ~9pm. I also see my achievements in local time using Firefox outside of Steam.